Electron beams are used in many industrial processes such as for drying or curing inks, adhesives, paints and coatings. Electron beams are also used for liquid, gas and surface sterilization as well as to clean up hazardous waste.
Conventional electron beam machines employed for industrial purposes include an electron beam accelerator which directs an electron beam onto the material to be processed. The accelerator has a large lead encased vacuum chamber containing an electron generating filament or filaments powered by a filament power supply. During operation, the vacuum chamber is continuously evacuated by vacuum pumps. The filaments are surrounded by a housing having a grid of openings which face a metallic foil electron beam exit window positioned on one side of the vacuum chamber. A high voltage potential is imposed between the filament housing and the exit window with a high voltage power supply. Electrons generated by the filaments accelerate from the filaments in an electron beam through the grid of openings in the housing and out through the exit window. An extractor power supply is typically included for flattening electric field lines in the region between the filaments and the exit window. This prevents the electrons in the electron beam from concentrating in the center of the beam as depicted in graph 1 of FIG. 1, and instead, evenly disperses the electrons across the width of the beam as depicted in graph 2 of FIG. 1.
The drawback of employing electron beam technology in industrial situations is that conventional electron beam machinery is complex and requires personnel highly trained in vacuum technology and accelerator technology for maintaining the machinery. For example, during normal use, both the filaments and the electron beam exit window foil must be periodically replaced. Such maintenance must be done on site because the accelerator is very large and heavy (typically 20 inches to 30 inches in diameter by 4 feet to 6 feet long and thousands of pounds).
Replacement of the filaments and exit window requires the vacuum chamber to be opened, causing contaminants to enter. This results in long down times because once the filaments and exit window foil are replaced, the accelerator must be evacuated and then conditioned for high voltage operation before the accelerator can be operated. Conditioning requires the power from the high voltage power supply to be gradually raised over time to burn off contaminants within the vacuum chamber and on the surface of the exit window which entered when the vacuum chamber was opened. This procedure can take anywhere between two hours and ten hours depending on the extent of the contamination. Half the time, leaks in the exit window occur which must be remedied, causing the time of the procedure to be further lengthened. Finally, every one or two years, a high voltage insulator in the accelerator is replaced, requiring disassembly of the entire accelerator. The time required for this procedure is about 2 to 4 days. As a result, manufacturing processes requiring electron beam radiation can be greatly disrupted when filaments, electron beam exit window foils and high voltage insulators need to be replaced.
The present invention provides a compact, less complex electron accelerator for an electron beam machine which allows the electron beam machine to be more easily maintained and does not require maintenance by personnel highly trained in vacuum technology and accelerator technology.
A preferred embodiment of the present invention is directed to an electron accelerator including a vacuum chamber having an electron beam exit window. The exit window is formed of metallic foil bonded in metal to metal contact with the vacuum chamber to provide a gas tight seal therebetween. The exit window in less than about 12.5 microns thick. The vacuum chamber is hermetically sealed to preserve a permanent self sustained vacuum therein. An electron generator is positioned within the vacuum chamber for generating electrons. A housing surrounds the electron generator. The housing has an electron permeable region formed in the housing between the electron generator and the exit window for allowing electrons to accelerate from the electron generator out the exit window in an electron beam when a voltage potential is applied between the housing and the exit window.
In preferred embodiments, a series of openings in the housing forms the electron permeable region. The exit window is preferably formed of titanium foil between about 8 to 10 microns thick and is supported by a support plate having a series of holes therethrough which allow the electrons to pass through. The configuration of the holes in the support plate are arrangable to vary electron permeability across the support plate for providing the electron beam with a desired variable intensity profile. Typically, the exit window has an outer edge which is either brazed, welded or bonded to the vacuum chamber to provide a gas tight seal therebetween,
The vacuum chamber preferably includes an elongate ceramic member. In one preferred embodiment, the elongate ceramic member is corrugated which allows higher voltages to be used. An annular spring member is coupled between the exit window and the corrugated ceramic member to compensate for different rates of expansion.
In another preferred embodiment, the elongate ceramic member has a smooth surface and a metallic shell surrounds the ceramic member. The ceramic member includes a frustoconical hole which allows an electrical lead to extend through the frustoconical hole for supplying power to the electron generator. A flexible insulating plug surrounds the electrical lead and includes a frustoconical surface for sealing with the frustoconical hole. A retaining cap is secured to the shell for retaining the plug within the frustoconical hole.
The present invention also provides an electron accelerator including a vacuum chamber having an electron beam exit window. An electron generator is positioned within the vacuum chamber for generating electrons. A housing surrounds the electron generator and has an electron permeable region formed in the housing between the electron generator and the exit window for allowing electrons to accelerate from the electron generator out the exit window in an electron beam when a voltage potential is applied between the housing and the exit window. The housing also has a passive electrical field line shaper for causing electrons to be uniformly distributed across the electron beam by flattening electrical field lines between the electron generator and the exit window.
Preferably, the electron permeable region includes a first series of openings in the housing between the electron generator and the exit window while the passive electrical field line shaper includes a second and third series of openings formed in the housing on opposite sides of the electron generator.
The present invention provides a compact replaceable modular electron beam accelerator. The entire accelerator is replaced when the filaments or the electron beam exit window require replacing, thus drastically reducing the down time of an electron beam machine. This also eliminates the need for personnel skilled in vacuum technology and electron accelerator technology for maintaining the machine. In addition, high voltage insulators do not need to be replaced on site. Furthermore, the inventive electron beam accelerator has less components and requires less power than conventional electron beam accelerators, making it less expensive, simpler, smaller and more efficient. The compact size of the accelerator makes it suitable for use in machines where space is limited such as in small printing presses, or for in line web sterilization and interstation curing.